1. Field of the Invention
The present invention relates to a camera system which causes a flash unit to perform a preliminary emission before a main emission, performs light measurement during the preliminary emission and computes a correct control value of the main emission on the basis of the result of the light measurement.
2. Description of the Related Art
Various camera systems have heretofore been proposed as a camera of the type which is capable of adjusting the amount of emission pointed at a subject so that correct exposure is automatically achieved. A system which is arranged to measure the subject-reflected light of the light emitted from a flash unit and obtain the amount of correct emission has been widely used because of its highly precise light measurement performance. One example of such system is a TTL flash control system arranged to find the amount of correct emission by measuring the film-surface-reflected light of light which reaches a film surface during exposure, and another example is a system arranged to perform a preliminary emission pointed at a subject and control a main emission so that the amount of emission of the main emission is made equivalent to the relative amount of emission with respect to the amount of emission of the preliminary emission.
In general, during flash photography, the light emitted from a flash unit is directly projected onto a subject. However, bounce flash photography is also often carried out by projecting the light emitted from a flash unit to a ceiling or the like and illuminating a subject with diffused light reflected from the ceiling or the like.
FIGS. 46(a) and 46(b) schematically show different examples of flash photography. FIG. 46(a) shows normal flash photography, and FIG. 46(b) shows bounce flash photography. In the bounce flash photography, a subject can be indirectly illuminated, so that it can be photographed with soft light.
For the purpose of flash photography intended to emphasize an three-dimensional effect, a flash unit may be made to emit light toward a subject not from the same position as a camera body but from a position away from the camera body. In this case, the flash unit and the camera body cooperate with each other to perform photography while exchanging information with each other in a wired or wireless manner.
In addition, for the purpose of close-up photography (macrophotography) of a subject, a ring type of flash unit having an annular flash illumination surface capable of being disposed around the periphery of a lens may be used so that a small subject can be photographed on an enlarged scale in the state of being illuminated with the light emitted from the flash unit.
However, in the conventional camera systems, in many cases, the amount of emission in flash photography is controlled on the assumption that the reflectance of a subject is similar to that of a gray sheet having a 18% reflectance. This leads to the problem that the amount of emission is controlled so that a white or black subject is photographed as a gray subject. In addition, if a regular reflection object, such as glass, is present on an image plane, the luminance of the subject-reflected light of flash light becomes extremely high, so that the amount of emission is controlled to be reduced to an underexposure level which is greatly low relative to a correct exposure level.
Japanese Laid-Open Patent Application No. Hei 4-33195 has proposed a system which is arranged to cause a flash unit to perform a preliminary emission before exposure, cause a multiple divided light measuring sensor to measure subject-reflected light, select a particular light measuring area on the basis of information indicative of the amount of emission of the preliminary emission and subject distance information, and perform a computation for correcting the amount of emission of a main emission, by using the measured light value obtained from the selected light measuring area.
This system provides control based on normal flash photography which is carried out on the assumption that the distance from a camera to a subject is equal to the distance from a flash unit to the subject. As a result, in the case of the aforesaid bounce flash photography, the flash photography which is performed in such a way that a subject is illuminated with flash light from a position away from a camera, or the macrophotography, the control of the selection of a light measuring area based on the distance information and hence the computation of the amount of emission of the main emission becomes meaningless, so that the amount of emission may be incorrectly controlled.
In addition, this system has the disadvantage that even if a photographer intends to control the amount of emission of the flash unit so as to correctly expose a subject nearer to the camera than an in-focus subject or intends to take an underexposure photograph, as by intentionally placing a regular reflection object, such as glass, or other high reflection objects in an image plane, the system will perform a uniform correction of the amount of emission or change light measuring areas to be used for control of the main emission, thereby providing flash photography which does not reflect the intention of the photographer.
In the conventional camera systems, if a subject is darker than a background under backlight conditions, the intensity of a stationary light component becomes higher than that of flash light. As a result, if the area in the image plane of a backlit main subject is smaller than the area of a light measuring sensor, a correct measured light value cannot be obtained, so that when a photographer slightly changes a composition, the state of exposure greatly varies and so-called exposure unevenness occurs.
Japanese Laid-Open Patent Application No. Hei 6-250253 and others have proposed that a decision is made as to whether a subject is backlit and a control method is varied on the basis of the decision. However, since there is only a combination of backlight and non-backlight, exposure unevenness still occurs under photographic conditions similar to backlighting conditions.
Many of the recent single-lens reflex cameras or the like which use focal plane shutters have been designed so that a photographer can select a flat emission mode for keeping the emission intensity of flash light approximately constant (flat) during the period from the instant when a shutter leading curtain starts running until the instant when a shutter trailing curtain completes running. The difference between the flat emission and the flash emission whose emission intensity has a peak is shown in FIG. 47. Although the flash emission only allows flash photography to be performed in the case of a shutter time which causes a shutter to be fully opened, the flat emission allows flash photography to be performed even in the case of a high-speed shutter time which uses slit exposure.
Since the flat emission takes a longer emission time than the flash emission as shown in FIG. 47, the guide number of the flat emission is smaller than that of the flash emission. For this reason, if the distance to a main subject is far or a large aperture value is used in photography, the flat emission may provide an underexposure compared to the flash emission.
Photographers who can understand the meaning of the guide number will be able to predict occurrence of an underexposure. However, beginners or the like who frequently make use of an automatic exposure mode during photography and do not fully understand the meaning of the guide number will have difficulty in making such a prediction. Some of the beginners may have to check warning displays of their cameras after the completion of photography using the flat emission and newly perform photography using the flash emission, or may forget to check such warning displays and note an underexposure after the development of photographs.
In the conventional TTL flash control system, the luminance of a subject is measured indirectly and vaguely only at a particular location in the image plane by making use of the diffused light reflected by a film surface. This leads to the problem that if the size or the composition of the subject varies, exposure becomes instable and continuous photography (continuous shooting) becomes difficult to perform with the same exposure level.
The above-cited Japanese Laid-Open Patent Application No. Hei 4-331935 has proposed the technique of causing a flash unit to perform a preliminary emission before exposure, performing light measurement of the preliminary emission, and correcting TTL flash control at the time of a main emission. However, if the preliminary emission is performed with the diaphragm of a photographing lens stopped down for the purpose of viewing a depth of field or the like, it is impossible to perform accurate light measurement, so that accurate correction becomes impossible or a complicated computation becomes necessary.
In addition, after the preliminary emission, if the photographer is worried about an overexposure or the like and deviates the direction of the emission part of the flash unit from the subject, it is impossible to achieve correct correction by using the measured light value obtained during the preliminary emission without modification.